Radio frequency (RF) transmitters used in mobile phones typically include a power amplifier, which amplifies the signals being transmitted from the phone's antenna. The power amplifier may be operated in its non-linear region near saturation for power efficiency. To avoid distortion of the transmitted signals due to the non-linearity, the signals may be digitally predistorted prior to digital-to-analog (D/A) conversion and up-conversion to the desired carrier frequency.
Digital predistortion may be performed by multiplying the modulated signals prepared for transmission by a set of predistortion values. The predistortion values are chosen such that the product values entering the power amplifier will be distorted by the power amplifier to return to a substantially linear amplification of the modulated signals. This may be achieved by selecting predistortion values that are the approximate inverse of the distortion produced by the non-linear power amplifier.
The predistortion may prevent the transmitter from transmitting signals on channel bands other than the band assigned to the transmitter. Accordingly, it would be advantageous to provide amplification in a linear range of a power amplifier that is adaptable to any channel band.